1.The tickets can be bought at the entrance of Sigiriya.It would cost you US$25 for a adult and US$12.50 for a child.
2.The travel time from Colombo to Sigriya would take approximately 5 hrs. Therefore, it is always advisable to go there one day before and lodge in near by hotel. So, you can explore the wonders of Sigiriya on the next day with enough time.
3.It is always advisable to start early in the morning, so that first you can climb the rock and descend before noon to avoid any discomfort due to heat of sun. Then you can explore the rest of the wonders leisurely as sun light is not a disturbance to walk through its gardens and water park.
4.If you are in a hurry you can finish things in Sigiriya approximately in 5 hrs, but my advice is to spend the whole day there and look for other stuff which I have not described above, such as caves close the Sigiriya rock with many more frescos which can hardly be seen.